Multilayer filter



Patented July 14, 1931 UNITED STATES.

PATENT OFFICE MBEBT I4. SEBASTIAN AID LEO arm, 01' EDGEWOOD, MARYLAND, ASSIGHOBB TO HARRY A. KORE, 0] EDGEWOOD ARSENAL, MARYLAND Io Drawing.

Application filed October 28, 1926. Serial Io. 145,685.

(GRANTED mu. m A01. 01 [ARCH 8, 1883, AS manna. nan. 80, 1928; 370 0.0. 757) The invention described herein may be manufactured and used by or for the Government for governmental purposes, w1thout the ayment to us of any royalty thereon.

This invention relates in general to filters and more particularl has reference to filters com osed of a lura 'ty of layers commonly use in gas mas Heretofore in the art among the various methods for removing smoke, gas or other foreign substance from the air numerous types of filters have been utilized. A number of fabrics and other materials have been tested for filtration qualities both in a single layer or in a plurality of layers. In practically every instance the resistance of the filters previously used has become abnormally high, when a high filter efliciency was obtained.

An object of this invention is to provide a treatment for multi-layerfilters deslgned to greatly increase the filtering efliciency without appreciably raising the resistance.

A further object of this invention is to provide an improvement for multi-layer' filters that will greatly increase the filtering efliciency and decrease the normal resistance.

With these and other objects in view which may be incident to the treatment of filters embodied herein, the invention consists in the process to be hereinafter set forth and claimed, with the understanding that the several necessary steps comprising the invention may be varied in their order or suitable ingredients substituted in place of those designated without departing from the spirit and scope of the appended claims.

In order to make the invention more clearly understood there is decribed hereinafter means for carrying the same into practical efli'ect, without limiting the treatments in their useful applications to particular ingredients, which for the purpose of explanation have been made the subject of illustration.

This invention consists in general of the impregnation of each section of a multilayer foraminous filter withsolid particles to eliminate the smaller pores, and thereby partially equalize the remainder, the impregnation to be uniform in all layers or to vary one layer from another. In some instances it is desirable to have a relatively high impregnation on the outer layer of the filter and a progresswely decreasing impregnation on the mner adjacent layers. It further consists of the provisionof a method for further equalizing the diameter of the pores 'of all the sections by increasing the number of layers to prevent the large pores coinciding and thereby eliminating the large pores with out raising the resistance to any considerable extent.

The theory of filtration from which the sub ect matter forming this inventionis derived may well be understood. When a gaseous stream containing smoke or other particles of material comes in contact with a filing a certain distance. That is, the time element enters and the longer the duration of contact of the particles-with the surface of the filter, the greater the probability that the partlcles will be deposited onthe same. It follows that two ways exist for a filter to be improved, either by increasing the length 'of the travel of the particle on the surface, or by increasing the time the particle takes in traveling over a given distance on the surface. The length of the path may be increased by decreasing the number of pores or increasing the distance between the pore openings.

The time the particle takes'to travel over a given path on the surface may be increased by raising the resistance of the filter. Since an increase in the resistance would retard the normal operation of the filter, the increase of the length of the path of the particle over the surface is therefore the logical improvement.

As the rate of How through a capillary tube is proportional to the fourth power of 9 filter, for in a random distribution of pores its diameter, the small pores-may be eliminated without increasing the resistance to any great degree and a great gain in filtering efiiciency will result.

These small pores may be eliminated by impregnating each filter layer. This is in efi'ect a plugging action not due to the clog- 4 ceases to Any soli ized solid material may be used to' accomplish gm of a pore by a single large particle but tot edepositionand formation of a flocculent aggregate at the mouth ofthe pore, so that the diameter is constricted and the air ass through that particular pore. d impregnatingsmoke or pulverthis step. -It is evident that by passin smoke or other substance throu h the filter the smaller pores will'be close This di- Inini'shes the number of, functioning pores and causes an increase of the length of the path necessary for the particle to travel. It is apparent that the natural j1mpre'gnation occurring in a filter during operation would not accomplish this result, in that the outer layer only would be so clogged or plugged, and while the peak of efiiciency is reached for a short time, there is a corresponding rapid increase of' resistance that retards the action.

It .is. manifest that since the resistance of the filter is proportional to the fourth power of thediameter of the pore, to accomplish a low resistance it is necessary to prd'vide pores proportion.

. thickness of the filters and necessarily in of as large a diameter as possiblewithout decreasing the efficiency. If the pores be of too great adiameter thefilter willnecessarily function improperly allowing substances 'to pass entirely through, thereby lowering its efficiency. It follows. that by the provision of pores of a critical-diametena filter may be increased in efliciency without raisingthe resistance much. By critical diameter meant a pore having the'largest diameter possible without decreasing the efliciency of the filter;- or raising the resistance; Thus 1f the pore diameter be decreased to a point be-.

low the critical. diameter, the resistance will increase very rapidly with no gain in efii-- Conversely by having poresof. too

ciency. great a diameter, while the resistance Wlll be decreased the efliciency will be lowered in ,The critical pore size may either of two methods, first by increasingthe creasing the resistance'faster than the gain in eflic'iency, or second by eliminating unnecessary pores, namely those very small and those very large. The'larger pores may be.

disposed of by increasing the layers of the having various size openings, laying one layer on another allows only a small degree of probability of the larger pores of the different layers coinciding. It will be appreciated that the larger the pore diameter, the

smaller is this probability. "A

thickness therefore can be reached when the pores are as large as or smaller than the.

.critical pore diameter. The smaller pores:

may be eliminated by the impregnation of the several layers with smoke or other substance adapted to-clogor plu themso that the diameter is constricted an the smoke orgas ceasesto' pass. As the greatest efiiciency of. the filter exists when the critical pore size'is obtained, it is necessary to determine that diameter. This may be accomplishel by impregnating a single layer of filtering materlal until a'certain-arbitrary degree of reg sistance is reached. A s'econdfilter is prepared composed of two.- layers and impregnated until a certain arbitrarydegreeofresistance is reached. Asecond filter is prepared composed oftwo layers and impreg nated until the combined resistance "of the two is equal to the arbitrary resistance taken as a standard; in this manner filters composedof layers varying in proportion are prepared. The conclusion of the series .is reached when the number of untreated layers required to give the standard. resistance is obtained.

Each of the filter 'units are then tested against the material towhich they are adapted tobe subjected where it will necessarilyfollow thatthesin e'la er-being of. high penetration will beef. ow ter eificiency due to the presence of largepores. As

the numberof-layers is increased the filtering efliciency will mount rapidly until'a cer-' .tain number of layers is reached, namely, un

til the large and small pores are eliminated and the filter in this stage possesses the maximum number of pore openings of the criti- 1 tion although this is arbitrary with t e need. obtained by Filtering material of an substance de-- sired may be used provide it is of proper texture tor the process. The matter used -for* impregnation may be any substance found most suitable. It has beenfound that camphor. black obtained by burning camphor, or acetylene black' by burning acety--.

lene, .or zinc oxide obtained from the decomposition of zinc ethyl are satisfactory sub stances. .It is understood that this invention is not limited to any particular material as an impregnant and that-further the method of impregnation may be accomplished 'by the use of finely pulverized substancesblown on the filter, drawn in by suction and and deposited or by any other of numerous conventional methods.

The construction of the filter .may be varied to suit the need. Long strips may be impregnated and wrapped to a' suflicient number oflayers around a screen or any desired object, or may be as preferable, out

to the proper size and each layer impregnated with equal or unequal amounts of impregnating material, and then built up in layers until the critical pore diameter is ob tained.

While there is shown and described the referred embodiment of this invention, it is to be understood that it is not confined to the precise details or treatment herein set forth, by way of illustration, as it is apparent that many changes and variations may be made therein, by those skilled in the art, without departing from the spirit of the in vention, or exceeding the scope of the appended claims.

' We claim:

1. A process for building a filter comprising the steps of se arately impregnating each of a plurality 0 layers of filtering material with finely divided solid particles adapted to fill up certain pores of said ma-- terial and superimposing the layers of said material upon each other so as to obtain a critical pore diameter.

2. A process for building a filter comprising the ste s of separately impregnating each of a plura 1ty of layers of filtering material with unequal amounts of finely divided particles adapted to fill up certain ores of said material and superimposing t e layers of said material upon each other so as to obtain a critical pore diameter.

3. A process for building a filter comprising the ste s of separately impregnating each of a plum ity of layers of filtering material with lamp black adapted to fill up certain pores of said material and superimposing the layers of said material upon each other so as to,obtain a critical pore diameter.

4. A process for building a filter comprising the ste sof separately impregnating each of a plum ity of la ers of fi tering material with zinc oxide a apted to fill up certain pores of said material and superlmposing the layers of said material upon each other so as to obtain a critical pore diameter.

5. A filter comprising a plurality of layers of filtering material superimposed upon each other, eac of said layers being impre ated with finely divided solid particles a apted to fill u certain pores thereof.

6. A ter comprising a plurality of layers of filtering material superimposed upon each other, eac of said layers being impregnated with unequal amounts of finely divided solid particles so as to fill up certain pores thereof.

7. A filter comprising a plurality of layers of filtering material superimposed upon each other, each of said layers bein impregnated with lamp black adapted to up certain pores thereof.

8. Afilter comprising a plurality of layers of filtering material superimposed upon each other, each of said layers bein impregnated with zinc oxide adapted to 1 up certain pores thereof.

In testimony whereof we afiix. our signatures.

ROBERT L. SEBASTIAN. LEO FINKELSTEIN. 

